I'm sure you're getting that from the many Reddit comparisons. But in every one of those I've looked at the one thing that they forget is that the PS4 is playing at 1080P and the steam deck is playing at 720P.

If you use the official dock on a 1080p monitor the steam deck starts to fall behind the PS4 rather quickly. It is true that on paper the steam deck Apu is getting close to the PS4 but that's without taking into account the TDP that the steam deck has set or the fact that steam deck is running at a lower resolution.

It gets even Messier when you take into account that not all games will run at 1080p on the ps4, some of them have Dynamic resolution support to maintain better performance at which point comparing the games becomes extremely difficult.

But if you just look at Absolute raw CPU and GPU compute numbers the steam deck isn't quite at the level of the PS4 which makes sense because it doesn't have access to quite as much power. It is much newer which is why it gets as close as it does. And if it had a 20 watt TDP I'm willing to bet it could beat it in most things, but as it stands because of the limitations of the cooling necessitating a lower TDP it's not quite there.

You are correct that arm is generally more power efficient but that's only the CPU, we don't know what type of Graphics they will be using and for a gaming device that's a fairly important piece of information. Nvidia doesn't really make mobile arm gpus anymore so they can't go with them, and if they go with one of the Arm based gpus those are significantly behind in terms of performance. They are good enough for your cell phone but they don't compare to AMD or Nvidia.